The present invention relates to an inflatable structure for supporting and reinforcing an object arranged in outer space, an array antenna with the inflatable structure, and a method of deploying the inflatable structure.
An object of an inflatable structure is to support and reinforce an object (for example, an antenna or the like) arranged in outer space. Such an inflatable structure has been researched and developed in recent years, and has been put to practical use.
An inflatable structure is formed by a sealed, bag-shape member. When the bag-shape member is folded up, a gas or the like is filled into an internal portion thereof, whereby internal pressure thus generated causes the member to reassume a tube shape, a balloon shape or other such desired shape.
In a case where the inflatable structure is of the tube shape, there are instances where the structure itself may serve as a prop or as a truss structure or other such unit member. Further, the inflatable structure can also be constructed as a square ring or as an annular ring tube, which extends in tension a membrane surface having an antenna element mounted on its inner side, to thereby support the antenna.
On the other hand, in a case where the inflatable structure is a balloon shape, the structure can also function as a part of a reflective mirror antenna, where the structure itself serves as a reflective surface.
A specific example of this type of inflatable structure will now be explained with reference to FIG. 17. FIG. 17 is a perspective view showing an antenna arranged in outer space, in a state supported by the inflatable structure.
As shown in FIG. 17, an antenna 100 is basically composed of an inflatable structure 101, a plane antenna 102 supported by the inflatable structure 101, and a plurality of tensioning cables 103 for supporting the plane antenna 102 with the inflatable structure 101.
The plane antenna 102 is composed of a disc-shape membrane surface with an antenna element mounted onto it. The inflatable structure 101 is a ring tube shape and is arranged surrounding the plane antenna 102.
Further, when the inflatable structure 101 is extended in tension, the plane antenna 102 is tensioned and extended by means of the tensioning cables 103. Accordingly, by being pulled in each different direction along the plane surface, the plane antenna 102 is supported by the inflatable structure 101 such that it maintains its surface geometry.
In a case where a reflective mirror is to be reproduced instead of the above-mentioned flat surface structure, a lens-type reflective mirror that is extended in tension in the interior space is also constructed using the inflatable structure.
A procedure for arranging the above-mentioned antenna 100 in outer space, such as in a satellite orbit, will now be explained.
First, the plane antenna 102 and the inflatable structure 101 are both stored inside a rocket fairing in their rolled or folded states.
Then the rocket is launched, and the antenna 100 is set on its satellite orbit. In this state, a gas or a urethane foam is filled into the inflatable structure 101 to deploy (restore) the inflatable structure 101 to its ring tube shape.
In this way, the plane antenna 102 which is in the rolled or folded state is extended, and the tensioning cables 103 pull uniformly on the membrane surface periphery of the plane antenna 102, to extend it into a flat plane without distortions.
Even when the inflatable structure described above is used with a large-scale structure to be arranged in out space, such as the antenna, the structure can be folded for example, so that the volume thereof can be reduced when it is launched using the rocket.
The above-mentioned structural characteristics are effective for putting equipment into satellite orbits when the mass (payload) that can be launched and the storage capacity of the fairing of the rocket are limited. Further, the characteristics are also effective for reducing the mass of each structure constituting an artificial satellite that will be put into satellite orbit and for reducing the cost of launching. Therefore, use of inflatable structures is being studied for a wide range of applications as a suitable structure for an antenna mounted on a satellite.
Demand to increase the size of structures arranged in outer space such as in satellite orbit has risen in recent years as space development has progressed.
For example, although the above-mentioned antenna has typically been a relatively small-scale antenna directly fixed and mounted on a low-orbit satellite, there is a demand to expand the diameter across its opening in order to increase its area gain so that it can be used as an antenna that is mounted onto a satellite that is in a stationary satellite orbit.
Further, its used is being studied for many applications, such as for high-speed data communications, radio wave observation satellites, synthetic aperture radar for remote sensing, and solar array mounting.
In order to make these applications practical, it is necessary to increase the size of the structures, including the antenna, the reflective mirror and the like. Further, regarding the antenna, there is a need to improve its area gain, side lobe level, directivity and other such electrical performance as well. Thus, a further increase in the precision level is necessary.
In order to satisfy these needs, examples of plane antennae and the like, which employ the inflatable structure, include one which is currently being developed in which the size of one side thereof exceeds 10 m. Moreover, an antenna in which the size of one side reaches several tens of meters is also being planned.
As described above, while there is a demand to increase the size of structures arranged in outer space, there is a need to reduce the mass and improve storability (compactness during storing) in light of restrictions on the payload of the launching rocket and the volume of the fairing where the structure is to be stored.
Therefore, there is a need to fulfill contradictory objects of increasing the size of the structure to be arranged in outer space and of reducing the mass of the structure and making it more compact when it is stored. Satisfying all these demands simultaneously entails technical difficulty.
In order to satisfy the demand for the reduced mass, it is necessary to employ a construction using a very strong and very elastic material, to increase the specific strength and the specific modulus of the inflatable structure.
On the other hand, when the size of the structure is increased, the overall flexibility of the structure increases, which means that even the slightest differences in the structure or in the manufacturing process result in distortions and deformations in the structure.
Further, the above-mentioned spaceborne structure must be very reliable regarding its structure and also with respect to environment. In a case where the bag-shape membrane that constitutes the inflatable structure of a tube shape is formed solely of a film, it has an advantage that it can be made lightmass and have a simple construction. However, in order to maintain the precision level of the structure, the gas pressure inside the inflatable structure must be controlled constantly.
Further, there is a danger that space debris and the like, which is also called space garbage, will cause damage to the membrane structure to cause loss of gas pressure, so that the structure cannot be maintained.
To prevent this, a curing-type resin is layered on these films, and sometimes reinforcing fibers are further layered on top of this, in order to enhance the hardness of the membrane structure. However, since these reinforcing measures increase the mass, there is a need for a structure which is both lightmass and highly reliable.
Thus, the following have been proposed as examples of structures which satisfy these various demands.
One is to increase its strength by using, as the rigidizing layer, aramid fibers or other such woven structure which is made airtight. Another example is to layer the aramid fibers or other such woven structure onto an airtight membrane to achieve a fiber-reinforced structure (FRP).
Here, the rigidizing layer for increasing strength includes several layers of reinforcing fiber weave. However, in this sort of layered structure, even though the strength and rigidity are increased, an increase in mass cannot be avoided. Further, these measures are not good for a structure which is to be folded up compactly and stored inside the fairing of the rocket.
Further, there is an inherent anisotropy in the weave of these reinforcing fiber weaves. Therefore, it is unavoidable that effects of this anisotropy will be present in the inflatable structure.
As an example of such a fiber-reinforced composite material, explanation will now be made of a typical biaxial woven fabrics of reinforcing fiber, making reference to FIG. 18. FIG. 18 is a plan view of the biaxial woven fabrics.
The biaxial woven fabrics 101a is composed of a woven structure in which reinforcing fiber bands 101y, 101x are woven together in an alternating fashion along the vertical and horizontal axes. Therefore, it possesses a structural anisotropy.
When this is used as the rigidizing layer of the inflatable structure, distortions and deformations caused by the anisotropy appear in the structure body. Particularly when the structural body is large, the influence of the anisotropy is increased due to the increased flexibility attendant on the size enlargement, and high-level precision cannot be maintained.
Thus, there are cases where it is necessary to layer the biaxial woven fabrics to eliminate the anisotropy; however, when a plurality of layers are laminated to achieve the necessary strength, the mass also increases, and thus it becomes difficult to achieve the reduction of the mass, which is the original object.
Further, as described above, during the launch it is necessary to roll or fold the structure to store it in the fairing; however, the structure having the thick, laminated layers of the reinforcing fiber weave inevitably has inferior storage properties.
On the other hand, in order to deal with these problems, when a highly elastic and very strong reinforcing fiber is used to reduce the density of the weave, the woven structure becomes loose and its rigidity suffers. Further, when the structure is rolled or folded up when stored in the fairing, misalignments occur in the woven structure, so the precision of the structure suffers and the intended precision level cannot be reproduced when the structure is deployed in outer space.
Further, when a highly elastic and very strong reinforcing fiber is used to decrease the number of layers of the biaxial woven fabrics, the anisotropy of the biaxial woven fabrics woven structure appears more evidently, which is not desirable.
An object of the present invention is to provide an inflatable structure which is lightmass and highly precise, and is adaptable to a demand for increased size.
A further object of the present invention is to provide an array antenna having an inflatable structure which is lightmass and highly precise, and is adaptable to a demand for increased size.
A further object of the present invention is to provide a method of deploying the inflatable structure with excellent reproducibility.
In order to achieve the above-mentioned objects, in accordance with the present invention, there is provided an inflatable structure provided with a multi-layer structure membrane comprising an airtight layer for forming an airtight space inside thereof; and a rigidizing layer formed with a triaxial woven fabrics of reinforcing fibers.
Therefore, since the rigidizing layer is formed with the triaxial woven fabrics of reinforcing fibers, it is easy to reduce the mass and improve the precision of the rigidizing layer. This is because it is not necessary to laminate layers to eliminate anisotropy since the triaxial woven fabrics exhibits a quasi-isotropy in its structure. Further, this eliminates a need to reduce the weave density.
A long shelf life prepreg resin may be used as a matrix resin in the rigidizing layer.
Here, the long shelf life prepreg resin refers to a resin which does not cure in a normal temperature environment.
Using the long shelf life prepreg resin as the matrix resin of the rigidizing layer as described above reduces a burden of managing the temperature.
The multi-layer structure membrane may have a protective layer for protecting the rigidizing layer, on an outer side of the rigidizing layer.
Accordingly, the strength can be improved further.
It is preferable that when the inflatable structure is deployed, the airtight layer exhibits a rate of deformation along its direction of deployment which is greater than a rate of deformation of the protective layer along its direction of deployment.
Accordingly, when the inflatable structure is deployed, the rigidizing layer is sandwiched between and pressured by both the airtight layer and the protective layer, so that adhesion among the layers is increased.
The protective layer is preferably provided with a gas escape hole.
Accordingly, since this enables gas that is enclosed in the rigidizing layer to escape, the adhesion between the rigidizing layer and the protective layer increases.
There are preferably provided a plurality of the multi-layer structure membranes formed in cylindrical shapes; and ring-shape rigid connecting members for linking the plurality of the multi-layer structure membranes.
Accordingly, the inflatable structure is easily formed.
When the inflatable structure is stored, the rigid connecting members preferably function as storage portions for storing the multi-layer membranes.
Accordingly, this enables the multi-layer membrane structure to be effectively protected.
The rigid connecting members are preferably constructed such that they can be connected with each other when the inflatable structure is stored.
Accordingly, this increases the rigidity during storage.
Further, in accordance with the present invention, there is provided an array antenna provided with the inflatable structure, comprising:
the above-mentioned inflatable structure;
a plurality of membrane members which are extended into a flat planar shape by deployment of the inflatable structure; and
a plurality of conductive non-woven fabrics serving as radiatied element, a conductive membrane serving as a grounding surface, and a microstrip line for supplying electricity to the conductive non-woven fabrics, which are each formed onto any one of the plurality of the membrane members,
characterized in that the membrane members are composed of triaxial woven fabricss.
In accordance with the present invention, since the membrane members are composed of the triaxial woven fabricss of reinforcing fibers, it is easy to reduce the mass and improve the precision of the membrane members.
It is preferable that the array antenna comprises two membrane members including a first membrane member and a second membrane member which are arranged in parallel, and in which:
the plurality of conductive non-woven fabrics are formed on a surface on an outer side of the first membrane member;
the conductive membrane is formed on a surface on an inner side of the second membrane member; and
the microstrip line is formed on a surface on an outer side of the second membrane member.
Further, it is preferable that the array antenna comprises three membrane members including a first membrane member, a second membrane member and a third membrane member, each arranged in sequence and in parallel, and in which:
the plurality of conductive non-woven fabrics are formed on a surface on an outer side of the first membrane member;
the conductive membrane is formed on a surface of the second membrane member facing the first membrane member; and
the microstrip line is formed on a surface on an outer side of the third membrane member.
Also, it is preferable that the array antenna comprises two membrane members including a first membrane member and a second membrane member arranged in parallel, and in which:
the plurality of conductive non-woven fabrics are formed on a surface on an outer side of the first membrane member;
the conductive membrane is formed on a surface on an inner side of the second membrane member; and
the microstrip line is formed on a surface on the inner side of the first membrane member.
It is preferable that the array antenna is further provided with:
a plurality of linear members provided linked to each of the members, which extend the membrane members into flat planar shapes by pulling each of the membrane members in respectively different directions along the flat plane, during deployment of the inflatable structure;
a binding member provided slidably with respect to each pair of the plurality of linear members, which allows a degree of freedom with respect to the spacing between the linear members when the inflatable structure is not in its deployed state, while it determines the spacing between each of the linear members when the inflatable structure is in its deployed state; and
a plurality of stoppers which are each fixed in predetermined positions with respect to each of the linear members and regulates the position of the binding member along its slide direction when the inflatable structure is in its deployed state, to thereby determine the relative positions of each of the linear members along the plane surface.
Accordingly, when the inflatable structure is not being deployed, a degree of freedom is allowed for the spacing between each of the linear members; therefore, the structure can be folded without obstructing the folding of the membrane members. On the other hand, when the inflatable structure is deployed, the positions of the membrane members are fixed along the height of the structure and along the flat plane of the structure so that the antenna function can be maintained.
Further, in accordance with the present invention, there is provided a method of deploying an inflatable structure that is comprised of a cylindrically shaped membrane with a multi-layer structure including an airtight layer that forms an airtight space inside, and a rigidizing layer that is provided outside the airtight layer and is formed of a triaxial woven fabrics of reinforcing fibers, the method characterized by comprising:
folding the cylindrically shaped membranes in advance in such a way that lines on the cylindrically shaped membrane which intersect a plurality of surfaces perpendicularly intersecting the cylinder axis each form a valley line, and neighboring valley lines along the direction of the axis perpendicularly intersect each other alternately at the points on the cylinder axis;
filling gas inside the airtight layer of the folded membrane to make the membrane into the cylindrical shape; and
curing the rigidizing layer in the state where the membrane has the cylindrical shape.
In accordance with the present invention, since the valley lines neighboring along the direction of the axis alternately intersect each other perpendicularly at the cylinder axis portion, a channel for gas passage is secured along the cylinder axis portion, so that the gas can be spread immediately throughout the entirety of the structure when the gas is filled in. Further, since there are formed no unpredictable wrinkles or the like other than the peak lines and the valley lines which are created when folding the structure, the reproducibility of the inflatable structure upon its deployment is excellent. Further, since the shape of the structure when folded itself exhibits a spring property which allows its deployment along the cylinder axis, the structure can readily unfurl when it is deployed.